In view of the recent environmental legislation that has established limits on the amounts of volatile organic compounds (VOCs) which are allowed in different coating systems, there has been a definite shift away from solvent borne coatings to water borne systems.
Many of the substrates to which conventional solvent-borne coating compositions have been applied are characterized by containing water-soluble staining agents. These stains are effectively blocked by solvent-based coatings because the stain is not solubilized by the coating solvent. However, when water borne coatings are applied to such substrates, these staining agents can leach from the substrate into the coating causing it to discolour.
Wood substrates, in particular, contain a number of water-soluble, chromophoric staining agents: of these, tannins and lignin can readily leach from the substrate into an aqueous coating, causing staining which appears as discolouration on the surface of the coating. Such leaching can occur upon application or during the service life of the coating. Moreover, other staining agents that can leach from wood are terpenoid based resins and alkaloids such as chlorophorin.
A staining of the substrate and of coatings applied to the substrate can also be caused by external sources. For example, cigarette smoke causes nicotine staining, which discolours light coloured coatings, and ink from pens can cause marker stains on the substrate. When such stained substrates are (re-)coated, an undesired discolouration of the top coat may occur.
Each of the above-mentioned effects of staining is highly undesirable in coatings and the patent literature consequently documents many attempts to improve the stain blocking, and particularly tannin-blocking properties of water borne coatings.
Historically, reactive pigments such as zinc oxide, aluminium zirconium phosphosilicate or barium phosphosilicate were found to be quite effective in blocking stains caused by inter alia tannins. However, these reactive pigments could obviously only be used in the sub-set of pigmented aqueous coatings and not in clear coatings. Moreover, these reactive pigments could cause de-stabilizing polymer gelation and coagulation in coatings containing them. As a consequence, it has become desirable to develop stain-blocking aqueous coating compositions which did not contain reactive pigments.
WO2005071023 (Nuplex Resins B.V.) discloses a stain blocking water borne coating composition, suitable for use in clear coats, which comprises an organic binder and, as the stain blocking agent, at least one type of inorganic nano-particles having a layered structure and a crystal structure with positively charged layers. In their natural or as-synthesized state these inorganic particles exist as aggregates consisting of a large number of primary particles. To provide haze free coatings, the inorganic particulates cannot be so aggregated and therefore, in the production of the coatings, the aggregates must be first broken down to the level of their primary particles or even further. This is achieved by combining a slurry of the particles with a suitable anionic dispersing agent but such a method step is time and energy consuming and reduces the formulation space of the coating composition. Furthermore, in practice, coatings containing these inorganic particulates have been found to be ineffective at blocking extractions from knots in wooden substrates.
In place of adding inorganic particulates to the aqueous coating composition, certain authors have proposed incorporating strong acids or cationic functionality into the backbone of the coating's binder to reduce or eliminate stain bleeding.
US Patent Application 2003/0073778 (Rohm & Haas) describes an aqueous stain-blocking coating composition which comprises from 0.1% to 10% by weight of at least one monomer bearing a pendant acid group having a pKa (in water at 20° C.) of less than 4, and salts thereof. The incorporation of such acid groups into the binder does however lead to an increased hydrophilicity of the coating which results in decreased water barrier properties.
U.S. Pat. No. 5,312,863 (Rohm & Haas) describes a stain-blocking coating composition containing an aqueous dispersion of a cationic latex polymer binder having an incorporated cationic functionality provided by an acid-protonated amine functional latex or a quaternary ammonium functionality. The principal drawback of this composition is the limited availability of coating ingredients that are cationic.
The present invention seeks to provide a stain-blocking aqueous coating composition which does not suffer from the disadvantages documented in the prior art.